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Carbohydrates

Definition and Classification

Carbohydrates, also called glucides (from the Greek "glucos" = sweet) are substances formed by carbon and water. They have a molecular form (CH 2 O) n and are mainly contained in foods of vegetable origin.

On average they supply 4 kcal per gram, even if their energy value fluctuates from 3.74 kcal of glucose to 4.2 Kcal of starch. Of these calories about 10% is used by the body for digestion and absorption processes.

Based on their chemical structure, carbohydrates are classified into simple and complex.

Simple carbohydrates, commonly called sugars, include monosaccharides, disaccharides and oligosaccharides. In nature there are more than 200 monosaccharides that differ in the number of carbon atoms present in their chain.

Hexoses (fructose, glucose, galactose) are the most important from a nutritional point of view.

Monosaccharides

GLUCOSE it is normally found in foods, both in free form and in the form of polysaccharide. It is the form in which other sugars must be transformed to be used by our body. Only 5% of the total amount of carbohydrates present in our body is represented by glucose circulating in the blood. Glycemic index = 100
FRUCTOSE it is found in abundance in fruit and honey; it is absorbed into the small intestine and metabolized by the liver which turns it into glucose. Its glycemic index is very low, equal to 23
GALACTOSE in nature it is not found free but linked to glucose it forms lactose, the milk sugar.

Oligosaccharides are formed by the union of two or more monosaccharides (maximum 10). They are found mainly in vegetables and in particular in legumes. The best known, since they are important from the nutritional point of view, are the disaccharides (sucrose, lactose and maltose).

Disaccharides

SUCROSE glucose + fructose; very common in nature it is present in honey, beets and sugar cane. Its glycemic index is 68 ± 5
LACTOSE glucose + galactose; it is the milk sugar and the least sweet of the disaccharides. Its glycemic index is 46 ± 6
MALTOSE glucose + glucose; little present in our diet is found mainly in beer, cereals and sprouts. Its glycemic index is 109

Among the oligosaccharides we mention the maltodextrins.

oligosaccharides

MALTODEXTRINE Maltodextrins are oligosaccharides deriving from the hydrolysis process of starches. They are used as energy supplements and can be useful in endurance sports. They supply energy in the short and medium term without over-straining the digestive system.

Polysaccharides are formed by the union of numerous monosaccharides (from 10 to thousands) through glycosidic bonds. We distinguish vegetable polysaccharides (starches and fibers) and polysaccharides of animal origin (glycogen). Polysaccharides containing a single type of sugar are called homopolysaccharides, while those containing different types of monosaccharides are called heteropolysaccharides.

Polysaccharides

STARCH it is the carbohydrate reserve of vegetables. It abounds in seeds, in cereals; it is also found in large quantities in peas, beans and sweet potatoes. In nature it is present in two forms, amylose and amylopectin. The higher the amylopectin content, the more the food is digestible.
FIBER they are structural polysaccharides, the most important of which is cellulose. Our body is not able to use them for energy purposes, but their fermentation in the intestine is essential to regulate the absorption of nutrients and to protect our body from numerous diseases. They are divided into water-soluble and not. The first chelane interfering with the absorption of nutrients, including cholesterol, the latter attract water by accelerating gastric emptying. The caloric contribution of fiber in the diet is zero.
Glycogen is a polysaccharide similar to amylopectin used as a source of storage and primary energy reserve. It is stored in the liver and muscles up to a maximum of 400-500 grams. The glycogen present in animals is almost completely degraded at the time of slaughter, so it is present in extremely small quantities in food.

Proportions of amylose and amylopectin in the starch molecule of different origin

Food

Amylose (%)

Amylopectin (%)

wheat

25.0

75.0

corn

24.0

76.0

rice

18.5

81.5

potatoes

20.0

80.0

tapioca

16.7

83.3

Digestion and absorption of carbohydrates

Carbohydrate digestion begins in the mouth where saliva enzymes begin to break down complex carbohydrates. In the stomach the action of salivary enzymes is interrupted by the acidic environment and resumes in the small intestine where, thanks to pancreatic juices (enzyme α-amylase), polysaccharides are reduced to monosaccharides.

While glucose is absorbed rapidly (both by osmotic gradient and by active transport), fructose is absorbed more slowly, through a facilitated diffusion mechanism, which is at the base of its low glycemic index.

CONTINUE: Functions of carbohydrates and role in Diet and Sport »

See also: Diet and carbohydrates

Artificial sweeteners